The 1/25 AMT '57 Thunderbird is easy to build but that also means fewer parts. The chassis is platform style with wire axles, drive train and exhaust molded in. It's an old mould so there's a little flash. Proportions are nice and it's fun for customizing. Custom options have varied over the years but most kits I've seen retain a few basic custom parts like wheel choices, custom top and some trim options. Kit instructions here.
The 1/24 Monogram '56 Thunderbird has more parts, more detail and no customizing options. Kit instructions here. I'm not aware of any serious fit issues from either of these that someone of your experience would trouble over.